* Divisional agency: Division of the country, over which the agency has usual operational jurisdiction.

The St. Louis County Police Department (SLCPD) is the primary and largest law enforcement agency serving St. Louis County in the U.S. state of Missouri. The current Chief of Police is Colonel Jon Belmar. According to the Charter of St. Louis County, the county police chief has all of the criminal law enforcement duties of the sheriff of St. Louis County, except for the operation of the St. Louis County Jail, which is handled by the St. Louis County Department of Justice Services (civilian), court bailiff and service of civil process. Court bailiff/civil process duties are provided by a court-appointed sheriff and his employees, none of whom have law enforcement powers.

This change in law enforcement services occurred in 1955, when the St. Louis County Charter was amended by the voters to restrict the duties of the St. Louis County Sheriff's Office. The position of elected sheriff was eliminated. St. Louis County is one of two counties in Missouri that does not have an elected sheriff. With the charter amendment, all law enforcement services were assigned to a new police agency, the St. Louis County Police Department. The department is primarily responsible for law enforcement and investigations within unincorporated St. Louis County and contract municipalities, however, it has full police authority throughout the county, including its 90 municipalities which are also served by 58 local police departments, some of them very small.[4] Additionally, St. Louis County police officers are authorized to enforce Missouri law in all counties in the state having a government of the first classification, which includes the independent City of St. Louis. Chief Jon Belmar was appointed to the position of Chief of Police on January 31, 2014 by the St. Louis County Board of Police Commissioners.

The St. Louis County Police Department was established in 1955. The department was created with 95 officers and 18 civilian employees.[5] The department is divided into four divisions: Patrol, Operational Support, Special Operations, and Criminal Investigation. It is the third largest local police agency in the state, the only county police department in Missouri and one of only a few in the United States. The SLCPD is accredited by the Commission on Accreditation for Law Enforcement Agencies. CALEA is the only international body that accredits law enforcement agencies. Only seven percent of all U.S. law enforcement agencies are accredited. In 2010, CALEA recognized the St. Louis County Police Department with it distinguished Tri-Arc Award. The Tri-Arc Award is reserved for those police agencies that have successfully accredited their law enforcement services, police academy and communications division. Only six agencies in the world have achieved this status.

From October 4, 1871, to October 6, 2016, the Officer Down Memorial Page reported that 11 officers in the St. Louis County Police Department died in the line of duty,[6] 5 of them from gunfire,[7] from automobile-related incidents, and 2 from heart attacks.[7] Many families of those killed in the line of duty get support from BackStoppers, a local charity.

The SLCPD is divided into seven police precincts, two of which are contracted cities, Fenton and Wildwood (5th and 6th). Each precinct's cars are issued plates with a letter corresponding to the assigned precinct. On November 1, 2010, the SLCPD took command of the City of Jennings Police Department under contract. The agency contracts with another 16 municipal governments to provide full-time police protection. Although the agency has full police authority of the sheriff in the entire county, including all municipalities, direct service population for the SLCPD is over 400,000 residents. A majority of these 400,000 residents are located in South St. Louis County and Affton, both of which are unincorporated but heavily populated and suburban. The division fields 33 School Resource Officers (SRO) to various school districts in the county. A volunteer Reserve Police Unit provides uniformed support to the division. The Division of Patrol is commanded by Lieutenant Colonel Troy Doyle.

Vehicles marked with plates "T" are Tactical Unit vehicles or are used for the Tactical Unit; vehicles with plates "E" are for administration, D.A.R.E., K-9 Units, MetroLink, or other special non-patrol units. [8][9]

In addition to Metro Air Support, the Division of Special Operations staffs a full-time Tactical (SWAT) Unit consisting of 21 members, as well as nine K-9 Units trained to detect narcotics, locate missing persons and apprehend suspects. As a matter of policy, the department uses SWAT and police dogs to serve all felony warrants.[11] Special Operations is also responsible for entrance security at the County Jail, Family Court and St. Louis County Courthouse in Clayton, the Crisis Intervention Team, DARE, Highway Safety Unit, Chaplains, MetroLink Unit, and the Office of Emergency Management. The Division of Special Operations is commanded by Lieutenant Colonel Michael Dierkes.

The St. Louis County Board of Police Commissioners is responsible for the operation of the department . The Board sets policy, makes promotions, holds both closed and open meetings and coordinates with the Chief of Police in providing police services to the citizens. Four of the five members of the board are selected by the County Executive, following approval of the County Council, with the County Executive serving as the sixth member.As provided in the St. Louis County Charter, the St. Louis County Police Department operates under the control of a civilian board of commissioners appointed by the County Executive with the approval of a majority of the County Council. Commissioners serve overlapping three year terms.